The speed of an elevator car in the elevator hoistway is often measured indirectly from the speed of rotation of the hoisting machine of the elevator. In this case a measuring error can arise, e.g. owing to elongation of the elevator ropes; also e.g. slipping of the ropes on the traction sheave of the hoisting machine causes a measuring error. Also the ungoverned movement of the elevator car resulting from breakage of the ropes cannot be detected by measuring the speed of rotation of the hoisting machine. If the position of the elevator car in the elevator hoistway is calculated by integrating the speed of rotation of the hoisting machine, the aforementioned errors of speed measurement are also transferred onwards into the position calculation of the elevator car. The accuracy of the measuring of the movement of the elevator car also affects e.g. the stopping accuracy of the elevator car.
The speed of rotation of the hoisting machine is usually measured with a separate sensor fixed to the hoisting machine, such as with a tachometer or an encoder. As mechanical components, sensors are susceptible to malfunction e.g. owing to vibration, dirt, temperature, etc. In many cases it would thus be advantageous to replace a speed feedback of the hoisting machine made with sensors with a solution that does not contain sensors. In such sensorless solutions the speed of rotation of the hoisting machine is determined e.g. on the basis of electrical magnitudes of the hoisting machine, such as on the basis of motor current and motor voltage. Eliminating sensors may, however, impair the measurement accuracy of the speed of rotation. For example, the rotor slip resultant from the operating principle of an induction motor affects the measurement accuracy of the speed of rotation of the rotor. Also accurate measuring of the speed of rotation of a synchronous motor can be difficult e.g. owing to measuring errors of motor current and motor voltage as well as to interference caused by the operation of a frequency converter.
The speed and position of the elevator car can also be determined e.g. by integrating the acceleration data of the elevator car notified by an acceleration sensor fixed to the elevator car. The aforementioned acceleration data of the elevator car notified by an acceleration sensor generally contains a measuring error to at least some degree, which is then transferred onwards to the speed information and position information of the elevator car.